In the spraying of functionally graded coatings, the particle ensemble delivered to the substrate varies from a relatively heavy, low-melting-point metallic particle to a significantly lighter, higher-melting-point ceramic particle. The desire is to deliver to the substrate a particle ensemble which has suitable velocity and temperature for the predictable and consistent formation of coatings with mixed particle types. The key to success is a thorough understanding of the relationship between spray gun parameters and the resulting particle condition. The gun parameters examined are powder loading (injection rate), powder mixtures, and secondary plasma gas (H 2 ). The spray characteristics measured were particle velocity, temperature, and spray pattern. The particle temperature and velocity are both significantly influenced by the flow rate of the secondary gas (gun power). The powder feed rate was found to have a small but measurable effect on both the spray pattern and the ensemble average particle temperature. It was observed that a "tight" hot particle spray pattern, unfortunately, does not necessarily minimize the number of cold unmelted particles.

When spraying is conducted in the ambient atmosphere, the entrainment of air cools down the plasma jet and affects its expansion. It may also cause the oxidation or the chemical decomposition of the sprayed materials. Inert Plasma Spraying (IPS), generally conducted in argon atmospheres, prevents these phenomena. However, the main drawbacks of IPS in comparison with air plasma spraying are the capital and apparating costs. To reduce the latter by 25 to 30%, nitrogen atmospheres may be used as a substitute for the conventional argon atmosphere. This paper presents a study in which titanium carbide and niobium powders were sprayed in argon and nitrogen atmospheres. Cryogenic cooling of the substrate was used during the spray process. This helps to maintain a low temperature in the chamber, produces thick coatings and allows the use of substrate materials that are sensitive to heat. The adhesion, roughness and microstructure of the coatings produced in both atmospheres are compared as well as their nitrogen content.

Experimental work has been undertaken to investigate the importance of the temperature of the substrate during deposition on the coating-adhesion of plasma sprayed borosilicate glass coatings. The work shows that the measured adhesion increases markedly with substrate temperature up to 400°C above which no further major increase takes place. Heat transfer and fluid mechanics calculations predict that the effect of substrate temperature is due to its influence through the cooling rate on the viscosity and flow of the molten glass particles as they impact on the substrate surface. The theoretical calculations also predict large temperature gradients through the thickness of the splats and glass coatings, and the consequent non-uniform thermal stress distributions are expected to contribute to the reduced splat retention rate and coating-adhesion at low substrate temperatures. The predictions were confirmed by an electron microscopy examination of the morphology of isolated splats, the deposits and the coating-substrate interface.

Phase transformations and/or decomposition of deposited compounds have an indisputable influence on materials properties of plasma sprayed deposits. Using water stabilized plasma, free-standing parts were manufactured from a mechanical mixture of zircon and alumina powders and annealed. The phase composition was determined by X-ray diffraction and the chemical composition was checked by x-ray microanalysis. ZrSiO 4 during plasma spraying decomposes into ZrO 2 and SiO 2 . In the as-sprayed condition, after a relatively fast quenching, the following phases can be found: a very fine eutectic mixture of tetragonal and monoclinic ZrO 2 , amorphous SiO 2 and a spinel phase of Al 2 O 3 . On annealing for 2 hours at 1300 and 1500 °C the spinel Al 2 O 3 transformed to corundum. At the same time, amorphous silica crystallized. Tetragonal ZrO 2 transformed to the monoclinic modification and together with SiO 2 formed again ZrSiO 4 . At the highest annealing temperature Al 2 O 3 and SiO 2 partialy reacted to form a small amount of mullite (3Al 2 O 3 .2SiO 2 ).

A search for cheap spraying materials offering interesting properties is conducted in connection with utilization of the high throughput water stabilized plasma. In this regard very promising materials are silicates. WSP PAL 160 was used for spraying garnets in their natural form and fused and crushed basalt. Chemical composition of both these materials is based generally on the same components but their contents are different. Paper reports on the spraying parameters used and then describes the structure and phase composition of deposits as well as their selected properties. Garnet of almandine type can be sprayed very well at a wide variety of parameters while pyrope type spraying is more difficult. Very interesting coatings were made of basalt.

A simple test procedure, based on steady state flow through a membrane, has been developed for measurement of the gas permeability of specimens over a range of temperature. The reliability of this equipment has been verified by testing solid disks containing single perforations and comparing the measured flow rates with those expected on the basis of laminar flow. Coatings of yttria-stabilised zirconia have been produced by plasma spraying in vacuum and in air. The specific permeability of these coatings has been measured at temperatures ranging up to 600°C, using hydrogen gas. It has been found that permeability is increased for coatings produced with longer stand-off distances and at higher pressures. Porosity levels have been measured using densitometry and microstructural features have been examined using SEM. A model has been developed for prediction of the permeability from such microstructural features, based on percolation theory. Agreement between predicted and measured permeabilities is good, although it is clear that more comprehensive data are needed in order to validate the model systematically.

The characterisation of the effectiveness of sealing was studied by metallographical investigations as well as comparing the investigations with respect to the corrosion- and wear behaviour of the used thermal sprayed coatings and last but not least by measuring of the insulation resistance of the coating system. The obtained results show that there are differences between the used sealants and it is possible through a mechanical treatment of sealed coatings to remove the sealants from the coatings. In the corrosion test the sealants show their efficiency. The sealants insulate the open porosity and prevent the corrosion attack owing to the interconnected pores.

A brief study was performed to examine the influence of starting powder composition, starting powder particle size and spraying environment on the chemistry, phase assemblage and porosity of Al-Cu-Fe plasma sprayed coatings involving a quasicrystaliine phase. It was found that a loss of Al during spraying results from the extremely low thermal conductivity of the quasicrystaliine phase in the starting powders. This loss changes the bulk composition of the deposited coating and partially controls the phases that develop. Smaller starting powder particles tended to lose more Al and, therefore, form less of the quasicrystaliine phase. Larger starting powder particles did not lose Al to the same degree, and produced coatings with more of the quasicrystaliine phase. However, these powders produced coatings also had a higher amount of porosity.

MoSi 2 provides good high temperature oxidation and corrosion resistance. However, the lower silicides such as MosSis do not provide such resistance. In this study, atmosphereic plasma sprayed (APS) MoSi 2 particle temperatures and velocities were measured under various torch conditions chosen to span the majority of typically utilized spray parameters. Empirical models of particle temperature and velocity were computed from the data. Three spray conditions were chosen to produce high, medium and low particle temperatures and velocities. Coatings produced under these spray conditions were characterized by profile tracing, quantitative x-ray diffraction, and SEM analysis. The Mo 5 Si 3 level in the coatings ranged from 5% to 8% while the Mo 5 Si 3 level in the starting powder was 0.6%. Particle size, particle trajectory, and torch parameters were found to be important factors in the Si loss process when APS depositing MoSi 2 .

The gas tunnel type plasma spraying enables to produce high quality ceramic coatings. Moreover a high hardness coating was obtained at a short spraying distance in the case of alumina coating. A high hardness zirconia (ZrO 2 ) coating could also be obtained at an atmospheric pressure by using the gas tunnel type plasma spraying. The Vickers hardness of the ZrO 2 coating at a short spraying distance was very high: a high hardness of more than Hv = 1200 was achieved at the surface side of the coating. In this study, the characteristics of these high hardness zirconia coatings produced the gas tunnel type plasma spraying were investigated by the measurement of the Vickers hardness of the coating. The microstructure of the obtained high hardness zirconia coatings were also investigated by the microscopic method and by the X-ray diffraction method.

The covering of titanium implants by means gas-thermal spraying of hydroxyapatite powders is an actual scientific, technical and medical problem. Application of hydroxyapatite for these purposes is more preferable. However, the problem of its structural and cyclic strength under conditions of bioenvironment response determines of application areas of such coatings and reliability of them usage. Structure and phase composition of hydroxyapatite coating under plasma spraying on titanium substrates and their changing, caused as conditions of forming coating on its increasing, so and conditions of spraying an laminar and turbulent plasma streem were studied. Exact belief about the crystalline structure and phase composition of coating is obtained by methods electronic microscopy and X-ray analysis. Changing of coating structure after sintering in the vacuum and electron beam melting in the vacuum is discussed.

High velocity oxy-fuel (HVOF) spray experiments were carried out using various spray systems. A comparison is made of the systems introduced as a first and second generation (Jet Kote, Diamond Jet, Top Gun, CDS) with the more recently introduced systems of the third generation (JP 5000, DJ 2600, DJ 2700). The comparison is based on particle velocities and experiments to evaluate the heat transfer to the particles. The results show that the systems of the new generation with a converging-diverging nozzle section can produce up to 50% higher particle velocities. The higher kinetic energy allows to reduce the thermal energy and to reduce thermally activated phase transformations of the coating material during the spray process. Carbide coatings produced with one of the new HVOF systems exhibit a higher density, higher bond strength and higher hardness as compared to coatings produced with one of the systems of the first and second HVOF generation. Furthermore, the reduced thermal energy yields less oxidative loss of carbon and opens the possibility to spray coatings with neutral or compressive internal stresses, a prerequisite to produce carbide coatings up to a thickness of several millimeters.

Spherical CoNiCrAlY powders (~45 + 5 μm) were sprayed using a Jet Kote II HVOF gun working with propane and oxygen. The coatings sprayed on two base materials: Inconel 625 (Ni base) and MarM 509 (Co base) were between 1,6 and 3 mm thick. The aim of the study was to examine the adherence problems in relation with this thickness and the imposed thermal cycles (thermal diffusion treatment and aluminizing). The studied parameters were the expansion coefficients evolution before and after the different treatments and the residual stresses between coating and substrate measured by incremental hole dilling method.

Pressure inside the spray chamber plays a key role during coatings manufacturing by thermal spraying and coating properties can be strongly affected by the selected pressure value. Spraying at low pressure results in a longer plasma jet length, higher particle velocity, lower coating porosity and higher purity and phase stability. For what concerns plasma-particle interactions, a reduction of pressure value drastically decreases heat transfer towards particles, therefore high power plasma equipment must be used to achieve a suitable melting degree of sprayed powders. Effects of low pressure values are well known, but few investigation have been carried out on effects of pressure for values higher than 1,000 mbar. In this paper a preliminary evaluation of pressure effects on plasma jet modifications, particle velocity and coatings microstructure is presented. By using the very innovative CAPS (Controlled Atmosphere Plasma Spraying) system, Ni-20%Al powders were sprayed at different pressure values, up to 3,600 mbar. The length and width of the visible part of the plasma jet was measured and controlled. Average particle velocity was also evaluated as a function of pressure. Coatings, manufactured on stainless steel substrates, were characterized by means of scanning electron microscopy and energy dispersive spectroscopy, x-ray diffraction and Vickers microhardness measurements. Results indicate that the higher the spraying pressure the lower the plasma jet length and particle velocity; but also a lower selective evaporation of aluminum and higher microhardness values were observed.

An important aspect of the APS plasma spray process is the turbulent mixing of the spray jet with the surrounding air. The air mixing into the jet causes undesirable oxidation of the sprayed coating. In this work the air mixing in the plasma jet was determined by direct measurement of the oxygen content. The measuring method is based on electrochemical determination of the oxygen potential using a solid electrolyte cell. The partial pressure of oxygen along the centreline of the plasma jet was measured with the hydrogen/argon and helium/argon ratio, the gas flow rate and the stand-off distance as experimental parameters. The oxygen content of the plasma tail flame was found to vary between 13.6 to 19.3 % depending on the hydrogen to argon ratio and the stand-off distance. Such high oxygen contents are far too high to avoid serious oxidation of metal coatings. The plasma spraying tests were carried out with WC-Co 17 coating powder. The plasma gases were Ar/H 2 and Ar/He. The respective oxygen contents by air mixing were measured to be 18.9 and 17.3 %. The WC-Co was observed to be decarburised more in the Ar/H 2 mixture than in the Ar/He mixture, which was attributed to the higher oxygen concentration, higher particle temperature and longer flight time in the plasma jet. Solid electrolyte cell technique was applied to this type of measurements and it proved to be a convenient way to determine the oxygen mixing in the plasma jet.